Membranes containing layers of silver ionomers of certain sulfonic acid containing polymers, especially fluorinated polymers, have been used to separate alkanes from alkenes. The separation of these types of compounds, especially those having the same number of carbon atoms, such as ethylene from ethane, propylene from propane, and pentene from pentane are often difficult by other methods, because for instance the boiling points of the alkane and alkene are similar, resulting in high energy requirements. This is especially true for lower boiling materials containing 2 to four carbon atoms, which would require cryogenic distillation, usually very energy intensive.
In membrane separation processes, thin film composite membranes are often used. Thin-film composite membranes (TFCs) usually consist of layers of dissimilar materials joined together to form a single membrane. This layered construction permits use of material combinations that optimize the performance and durability of the membrane. The same is true of alkane-alkene separation processes using silver ionomers, described herein are new TFCs for such separations.
As a minimum requirement one might believe that a membrane having a single layer, a “separation layer” (SL), of silver ionomer would be useful. However, such a type of membrane has two important drawbacks, the silver ionomer is expensive, and in thicker membranes which are required for the membrane to have sufficient strength, the permeance to the alkene, the amount of alkene which can pass through the membrane per unit time is relatively low. Therefore one (or more) thin separation layers are required for practical membranes. A composite membrane with another layer of material which physically supports the entire TFC, and which is laminarly contacting the separation layer can be added. This other layer is nonporous and the material passing through the TFC (in this case one or more alkenes), should also preferably diffuse through this other layer rapidly. Herein this other layer is called the high diffusion rate layer (HDR).
The use of various composite membranes with silver ionomer separation layers for the separation of alkanes from alkenes is known, see for instance A. van Zyl, et al., Journal of Membrane Science, 133, (1997), pp. 15-26, O. I. Eriksen, et al., Journal of Membrane Science, 85 (1993), pp. 89-97, and A. J. van Zyl, Journal of Membrane Science, 137 (1997), pp. 175-185, and U.S. Pat. No. 5,191,151. However none of these references describe a TFC in which a separation layer of a silver ionomer is used in combination an HDR layer described herein.
Certain layers in TFCs, layers, sometimes termed “gutter layers” are used, and are generally described in M. Kattula, et al., Designing ultrathin film composite membranes: the impact of a gutter layer, Scientific Reports, 5, Article Number 15016 (2015). There is no specific information concerning alkane-alkene separations using silver ionomers.